The present invention relates to an electrometer probe for measuring the electrostatic potential on a moving surface such as a photoconductive drum in an electrostatic copying machine. The measured potential may be used to control a developing bias voltage.
An electrode is disposed closely adjacent to the surface of the photoconductive drum to sense the potential of an electrostatic image thereon. A voltage is induced in the electrode which is proportional to the electrostatic image potential and is fed to a computing circuit which generates a developing bias voltage in accordance therewith. The bias voltage which is applied to a developing unit is slightly greater than the potential of the background areas of the electrostatic image and prevents the formation of a gray background in the finished copy. Preferably, more than one electrode is provided and the minimum output of the electrodes is selected since it corresponds to the background area potential.
This type of electrometer is known as the capacitance division type since capacitances exist between the drum surface and electrode and also between the electrode and ground. The induced voltage corresponds to the capacitance division ratio. The electrode is connected to an input of an amplifier.
It is important that the induced voltage in the electrode follow the potential on the surface without lag or drift. In addition, the electrode should not disrupt the electrostatic pattern on the surface.
The drift can be reduced to a low value as disclosed in U.S. Pat. No. 3,898,001 by surrounding all but a portion of the electrode which faces the surface with a conductive shield or guard. An improvement is disclosed in U.S. Pat. No. 3,729,675 in which a dielectric or electrically insulating material is provided between the detecting electrode and guard and the output of the amplifier is connected to the guard.
However, a problem which is not solved in these prior art disclosures is zero drift, which makes it necessary to set the zero point prior to each measurement. Zero drift is caused by current flow between the detecting electrode and guard due to a potential difference therebetween. This exists even if the amplifier is operated as a voltage follower with theoretically unity gain.
Assuming that the gain is 0.99999 and the input voltage is 300 V, a potential difference of 3 mV will exist between the electrode and guard. In addition, a potential difference caused by amplifier offset voltage can typically range from 0.5 mV to more than 5 mV. In order to minimize zero drift, the input resistance of the amplifier as viewed from the detecting electrode must be very large, the input capacitance must be very small and the input current must also be very small. However, an apparatus in which these conditions have been realized sufficiently to reduce zero drift to a negligible value has not been heretofore attained.